1. Field of the Invention
This invention relates to photosensitive materials and, in particular, to photosensitive materials including silicon atoms.
2. Art Background
Photosensitivity has been found in polymers having an all silicon backbone such as in A) linear poly(diorganosilylenes), sometimes called polysilylenes or polysilanes, i.e., materials having the general formula [R.sup.1 R.sup.2 Si].sub.n with R.sup.1 and R.sup.2 being various alkyl or aryl substituents and having n typically larger than 20, and B) in polysilyne network materials, i.e., materials having at least 70% of their silicon atoms bound to only one organic substituent and to three other silicon atoms.
Irradiation of linear polysilylenes with U.V. or deep U.V. light generally causes fragmentation that results after development in positive images--the unexposed regions remain after development. The photoreactivity of polysilynes is markedly different from that of polysilylenes. The polysilyne layer is exposed to ultraviolet light in the presence of oxygen to induce photooxidation with formation of crosslinked Si--O--Si networks. Such photooxidation produces a change both in solubility and in the refractive index of the oxidized relative to the unoxidized regions. The photooxidation allows selective removal by suitable solvents or halogen-based plasma reactive ion etching of the unexposed region to produce a negative image. Appropriate use of the refractive index change (if oxidized material is not removed) yields light guiding structures. Thus, photooxidation processes in polysilynes are suitable for fabrication of optical and electronic devices. (See U.S. Pat. No. 4,921,321, dated May 1, 1991.)
Soluble organosilicon films of partially characterized structure (reported in M. W. Horn et al, Journal of Vacuum Science and Technology, B8, 1493 (1990), to contain substantial Si--C--Si backbone bonds and an insignificant presence of Si--(Si)--Si bonds) have been deposited by plasma reaction of various volatile organosilicon compounds such as tetramethylsilane. These materials show a decrease in solubility and increased resistance to gaseous HBr or chlorine plasma etching after exposure to light at 193 nm with sensitivities of approximately 50 mJcm.sup.-2, but are essentially transparent and not useful at longer wavelengths such as at 248 nm. For many processes such as the formation of electronic and optical devices, photosensitive materials (denominated resists) having a photosensitivity better than 200 mJcm.sup.-2 and preferably better than 100 mJcm.sup.-2 at or above 248 nm are required to avoid undesirably long exposure times. (Photosensitivity is defined as exposure dose required to allow the development of an imaged film capable of functioning as an effective etch mask for subsequent pattern transfer by reactive ion etching.) Furthermore, photosensitive materials requiring 193 nm light involve processing complexities which make it currently impractical.
Soluble polymeric methylhydridosilylenes have also been prepared (see, for example, U.S. Pat. No. 4,537,942, dated August, 1985, D. Seyferth and H. Lang, Oganometallics, 10, 537 (1991), J. F. Harrod, "Inorganic and Organometallic Polymers") and in U.S. Pat. Nos. 4,719,273 and 4,537,942 their use, without further explanation, for photolithography, is suggested. No comment on properties indicating suitability for resist applications was provided beyond the basic suggestion that the materials are soluble in common solvents. In this regard, the use of a resist generally depends on solubility characteristic allowing deposition of solid films from solution by spin casting or spraying and allowing the solvent to evaporate. However, it has long been a goal to form a suitable resist on a substrate by deposition from the gas phase, useful for conventional deep to mid-UV photolithography (for example, at 248, 310, or 365 nm). Such gas phase deposition is advantageous since resist formation, exposure, development, and pattern transfer would become possible within an interconnected series of chambers (sometimes called a cluster tool) without degradation resulting from exposing the wafer to the ambient.
A suitable resist useful as a photodefinable glasslike reactive ion etch mask based on silicon chemistry having acceptable sensitivities at wavelengths longer than 200 nm that is capable of being deposited from the gas phase is not presently available.